Apparatus for isotropically stretching a plastics sheet

ABSTRACT

The invention provides an apparatus for isotropically stretching a plastics sheet in which the sheet is gripped by a plurality of grippers, which are disposed in an array of a particular configuration, e.g. a circle or a square. In order to stretch the sheet the grippers are moved outwardly on screw-threaded rods which are geared together, e.g. by bevel gears, at a rate to maintain the same configuration.

United States Patent Michel Salmon Lyon. France 715.388

Mar. 22. 1968 Aug. 3, 1971 Rhone-Poulenc S. A. Paris, France InventorAppl. No Filed Patented Assignee APPARATUS FOR ISOTROPICALLY STRETCHINGA PLASTICS SHEET 6 Claims, 6 Drawing Figs.

1.1.8. Cl 18/1 FS, 18/19 R Int. Cl B29c 17/00 Field of Search 18/1 FS;

[56] References Cited UNITED STATES PATENTS 2,752,630 7/1956 Taylor 18/1FS 2,912,716 11/1959 Frownfelter et a1. 18/1 FS Primary Examiner-AndrewR. Juhasz Assistant Eraminer- Leon Gilden Att0rne \'-Cushman, Darby &Cushman ABSTRACT: The invention provides an apparatus for isotropicallystretching a plastics sheet in which the sheet is gripped by a pluralityof grippers, which are disposed in an array ofa particular configuratione.g. a circle or a square. In order to stretch the sheet the grippersare moved outwardly on screwthreaded rods which are geared together,e.g. by bevel gears, at a rate to maintain the same configuration.

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APPARATUS FOR ISOTROPICALLY STRETCHING A PLASTICS SHEET The presentinvention relates to an apparatus for the uniform and substantiallyisotropic discontinuous stretching of sheet of plastics material, andprocess for stretching the sheets employing the apparatus.

Various apparatus have been constructed for the discontinuous stretchingof sheets of plastics material. Some effect a biaxial stretching in twoperpendicular directions; they result in anisotropic structures. Otherseffect stretching along multiple axes which are not uniformlydistributed nor strictly defined; they result in irregularly anisotropicstructures. It is often necessary to stretch articles in a uniformfashion in order to given them substantially equal mechanical,electrical and other properties in all directions.

Apparatus having already been constructed for stretching sheets alongconcurrent axes, resulting in articles which are substantially isotropicover a portion of their surface, but this substantially isotropicportion is always fairly limited. In addition, these apparatus are notalways convenient to use and more particularly are not well adapted tolaboratory needs. In fact, in some apparatus, the stretching meansremain within a treatment chamber, which often results in difficultiesin gaining access to the sheet when positioning it or when particularoperations have to be carried out. This arrangement also results in longtreatment times when it is necessary to operate successively at greatlydiffering temperatures. Apparatus have also been constructed which canbe introduced into a treatment chamber, but the stretching means usedand their arrangement result in heavy and bulky apparatus having regardto the dimensions of the stretched sheets, which involves the use oftreatment chambers of large dimensions and considerable heating means.Furthermore these apparatus are equipped with only a small number ofgrippers to hold the sheet of plastics material, so that a considerableportion of the surface is not stretched uniformly and cannot be used.

The invention has as its object to provide an apparatus, which isparticularly suitable for the needs of laboratories or testingdepartments, for the uniform and substantially isotropic stretchingofsheets of plastics material.

The invention provides an apparatus for isotropically stretching a sheetof plastics, such apparatus comprising a plurality of screw-threadedrods mounted withtheir axes extending radially from a central member, ascrew-threaded gripping means for the sheet mounted on each of the rods,the threads on the rods cooperating with the threads on the grippingmeans, the gripping means being disposed relative to one another in anarray of a particular configuration and gearing interconnecting the rodswhereby rotation of one of the rods will cause rotation of the remainingrods so that the gripping means move along their respective rod at arate to maintain the configuration constant.

The construction of such an apparatus is simple and, having regard tothe dimensions of the stretched sheets, the dimensions and weight of theapparatus are small, which makes it possible to use the apparatus intreatment chambers of small size.

Suitably there are at least five gripping means, and the gearingcomprises a bevel gear. A number of arrangements of bevel gears can beemployed; examples of these are a. a bevel gear is mounted at the innerend of each rod and cooperates with a single bevel gear mounted on thecentral member.

b. a bevel gear is mounted at the inner end of each rod and cooperateswith the bevel gears on each of the adjacent rods, the pitch of thescrew thread of each rod being opposite to the pitch of the threadofeach adjacent rod.

c. a bevel gear is mounted at the inner end of each rod and cooperateswith two idler gears, the idler gears each cooperating with the bevelgears on each of the adjacent rods.

The invention will be more fully understood from the followingdescription given merely by way of example with reference to theaccompanying drawings, in which:

FIG. 1 is a schematic side elevation of one embodiment of apparatusaccording to the invention in a half view section.

FIG. 2 shows a partial top plan view ofthe apparatus of FIG. 1.

FIG. 3 is a view similar to FIG. 1 of another form of the apparatus.

FIG. 4 is a view similar to FIG. 2 of a third embodiment.

FIG. 5 is a view similar to FIG. I ofa further embodiment.

FIG. 6 shows a partial top plan view of the apparatus of FIG. 5.

Referring to FIGS. land 2, a frame 1 carries a hollow hub 2 and a rim 3coaxial with the hub to give a rigid assembly; attached to the rim is ahandle 21 to permit easy manipulation of the apparatus. Around the hubare arranged, perpendicular to the axis of the hub, l2 sideways 4 andabove them 12 screwthreaded rods 5, the slideways and rods beingarranged in pairs, parallel, and in close proximity, to each other. Eachrod carries a screw-threaded carriage 6, which has a hole therein sothat the carriage can slide up and down the slideway. The carriage has agripper 8 for the sheet to be stretched, the gripper consisting of twojaws 9 and 10 joined by a pivot 11 and clamped to one another by a screw12. Each rod is seated in two bearings, one 13 in the rim and the other14 in the hub, so that the rod can rotate freely. The rods each carry attheir inner ends a bevel gearwheel 15, which cooperates with anotherlevel gearwheel 17 which is mounted on two bearings 18 and 19 situatedin the bore of the hollow hub 2, so that when the rod is rotated, thebevel gearwheel 17 rotates on an axis at right angles to the axis ofrotation of the rod thus driving the other rods. At the rim end of oneof the rods is mounted a coupling device 16 which cooperates with ageared motor (not shown) to enable the rod to be rotated so that thecarriages on that rod and on all the other rods can be movedsimultaneously and at the same speed up and down the slideways. Thetravel of the carriages is limited at both ends by limit switches 22.

FIG. 3 shows a variant of the apparatus of FIGS. 1 and 2 which isidentical to the latter except that there are two, instead of one,slideways, 4 and 5, one above and the other below the rod 5, and thatthe carriage has two holes, one for each slideway.

FIG. 4 illustrates an apparatus for treating a square-shaped plasticssheet. The carriages are arranged in square array and the pitches of therods are proportional to the distance of its carriage from the center ofthe square.

In FIGS. 5 and 6, there is shown a variant of the apparatus of FIG. 3,which instead of having a bevel gearwheel situated in the bore of thehub, it has the bevel gearwheels mounted on each rod cooperating withthose gearwheels on the adjacent rods. In order that all the carriagesmay be driven in the same direction, the pitch of the thread on each rodis opposite to that ofits neighbors.

Another variant consists in arranging an intermediate bevel gearwheelbetween two neighboring bevel gearwheels 15. It is then possible toarrange an even or odd number of screwthreaded rods which all rotate inthe same direction. The diameter of the intermediate or idler bevelgearwheel is not of critical importance; it is preferably less than orequal to the diameter of the bevel gearwheels mounted at the end of therods. Thus the apparatus is equipped with two or three series ofgearwheels which are arranged in a single circle or in two or threesuperposed circles coaxial with the hub.

The carriages are provided with sliding means constituted by smoothrings of bronze or ball bearing sleeves which slide along theseslideways. The holding jaws of the gripper have their opposite facesgrooved and one of thejaws is fixed and is integral with the carriageitself.

The frame is preferably plane and constituted by a rigid disc of sheetsteel. The slideways are cylindrical steel rods.

In order to obtain completely satisfactory operation, it is preferablefor the parts to be machined with precision, particularly the hub andthe gearwheels. The manufacturing tolerances of these parts have to beall the more carefully observed if the apparatus has to be subjected togreater temperature ranges. The angular positions of the screw-threadedrods and the slideways, and the parallelism of the slideways with thescrewthreaded rods are preferably precisely defined. The necessary playwill be allowed to permit the slideways and the screw-threaded rods toslide in their respective bearings at the rim end and thus to expandfreely.

The apparatus is preferably free of any device using balls such as aradial bearing, thrust bearing, sleeve etc., making it possible to avoidusing lubricant with the risk of decomposition of such lubricant andfouling the plastics materials during the course of treatment when it isnecessary to work with high temperatures. The parts subjected tofriction can be specially treated, for example by a Sulfinuz process.

The toothing of the bevel gearwheels is generally ground, likewise thescrew threads. The slideways, the screw threads and the frame arepreferably made with the same material, for example steel, in order tolimit the respective sliding due to expansion.

When the apparatus if provided with a central gearwheel, the latter canbe formed with a coaxial hole permitting inter alia the introduction ofa device for measuring the temperatures in the vicinity of the filmbeing stretched.

When the apparatus has no central gearwhecl, the gearwheels on the rodscan be arranged on the radially inner side of the bearings 14; butpreferably they will be arranged between the bearings 13 and 14, againstthe hub, in order to free the central funnel of the hollow hub of allmechanical elements. Generally the pinions 15 are identical and all meshwith one another; the driving forces are then automatically distributedat any instant in a balanced manner between the driving gearwheel andthe gearwheel exerting the highest resistant torque, whatever theirrelative positions. This arrangement is particularly advantageous since,owing to the excellent distribution ofdriving forces, it is notnecessary to overdimension the driving elements and furthermore thedriving of the carriages is controlled in a flexible and gentle manner.It is rendered easier by the use of two slideways on either side of thescrew-threaded rod.

The apparatus according to the invention can be used in the followingmanner:

The position of the carriages 6 is regulated so as to receive on thelower jaws the plastics sheet which is to be stretched and which haspreviously been cut to a circular shape. The jaws 9 and 10 of theclamping devices are opened, the plastics sheet is placed in position,and then the upper jaws are closed on the edges of the sheet which isgripped in the jaws by means ofthe screw 12. The sheet to be stretchedbeing thus held, the apparatus is introduced into a heating chamberpreviously taken to a suitable temperature. Forced circulation of hotair permits obtaining the desired thermal equilibrium rapidly, and auniform distribution of temperatures in the plastics material sheet.When the specific temperature for stretching is reached, by any suitablemeans, for example by means of a geared motor set situated preferablyoutside the heating chamber, and by means of the coupling device 16, oneof the screw-threaded rods 5 is driven. The grippers 8 carried by thecarriages 6 move away equally and progressively, whilst remaining onconcentric circles of increasing diameter. The driving speed of thecarriages 6 is regulated to a suitable value to ensure that the sheet isuniformly stretched without tearing, and this movement is continueduntil the desired extent of stretching has been achieved.

if the apparatus is of the form illustrated in FIG. 4, the grippers 8are located in an array having the configuration of a square. As therods 5 are rotated the pitch of the threads therein is such that thecarriages 6 and gripper 8 move outwardly to maintain the squareconfiguration, the size of the square, ofcourse, increasing.

The apparatus can subsequently be taken out of the heating chamber and,after the cooling of the plastics material sheet, the jaws can beunclamped by means of the screws 12 and the stretched sheet withdrawn.

The apparatus according to the invention can be used horizontally sothat the sheet to be stretched is arranged horizontally at its upperportion. The apparatus can also be used, if necessary, in any otherposition, and more particularly the sheet to be stretched can bearranged vertically, or horizontally but at the lower portion of theapparatus, which has been turned round. This makes it possible, wherenecessary, to immerse the sheet easily in a liquid bath in order toobtain, for example, stretching in a particular medium or a very rapiduniform distribution of temperature. The arrangement of all thestretching means of the apparatus on one side of the plane of the sheetto be stretched renders the sheet easily accessible.

The apparatus according to the invention is particularly suitable forlaboratory or testing departments work, but it can easily be made to anysize and of various materials so as to meet any industrial needs.

Many modifications may be made, for example:

The devices for clamping the sheet of plastics material can beconstituted by self-locking grippers. Their opening and closing can alsobe controlled by any automatic devices, such as mechanical, or pneumaticdevices.

It is often advantageous, more particularly in cases where considerableforces have to be applied in order to stretch thick panels, to guide thecarriages by means of two slideways arranged on either side of thedriving screw (see FIG. 3). It is also possible to guide the carriagesby means of two parallel driving screws which thus act as slideways. Inthe case of one embodiment with two slideways or two parallel screws itis possible, if desired, to dispense with the frame 1 and thus tolighten the apparatus whilst retaining adequate rigidity.

if desired, more particularly for stretching very strong films, anapparatus can be used which is provided with several coupling devicesdriven by a plurality of motors or by a motor and a transmission meansof suitable type, and each controlling the rotational movement of agroup of several gearwheels meshing with one another, each group ofgearwheels being arranged on a sector of one and the same circle.

It is thus possible to construct an apparatus of noncircular form, forexample square, rectangular, elliptical etc. (see FIG. 4). The carriagesprovided with clamping devices are arranged on the screw-threaded rodsin the selected form, for example a square. The carriages are driven byscrew-threaded rods whose pitches are proportional to the respectivedistances of the carriages from the center of the square. The carriagesare then displaced in accordance with concentric squares. in the samemanner, it is possible to stretch in accordance with other geometricfigures.

It is also possible to move the carriages provided with clamping devicesalong concurrent axes which are equally distributed along thegeneratrices of a cone, in which case the plastics material sheet can bestretched whilst resting on a fixed or mobile member. This arrangementmakes it possible, for example, to carry out or facilitate a wrappingoperation, or to stretch plastics material films which are not plane.Naturally, in this case the frame and the other elements are to be givenan appropriate shape.

The apparatus according to the invention permits the stretching ofplastics material, more particularly thermoplastic materials in the formof films sheets, skins, diaphragms, fabrics or panels. Stretching can becarried out at ambient temperature or in the hot state up the thehighest temperatures compatible with the products being treated, ingases or in liquids.

It is possible to carry out successive heat treatments at differenttemperatures, for example using successivelya plurality of heatingchambers the temperatures of which have previously been regulated toappropriate values. It is also possible to carry out various heattreatments or cold treatments such as coating or spraying. during thecourse of stretching. The apparatus according to the invention makes itpossible more particularly to carry out in the laboratory or workshoptests on samples for the purpose of studying their various mechanical,physical or chemical properties. but it may also be constructed withlarger dimensions for the uniform stretching of sheets or panels ofconsiderable surface area or considerable thickness. Finally, theapparatus can be adapted to fit into a production line, for example inorder to carry out wrapping operations.

The apparatus according to the invention permits the uniform andsubstantially isotropic stretching of a very large portion of theinitial surface of sheets of plastics material and allows easy access tothe sheet being stretched to facilitate manipulations or treatmentswhich have to be carried out. The apparatus is simple, light, compactand robust in construction, and can be introduced easily intoconventional heating chambers, which enables successive treatments atgreatly differing temperatures to be carried out rapidly, the apparatuswithstanding the highest,temperatures compatible with the products beingtreated.

EXAMPLE 1 An apparatus of the type shown in FIGS. 1 and 2 is used; itsdimensions are as follows: diameter: 360 mm., height: 63 mm. (88 mm.with handles), weight: 7.950 kg. It can receive films of circular shapeof at least 85 mm. in diameter and stretch them in circles reaching 290mm. in diameter. The ratio of the film surfaces after and beforestretching reaches 10, and the apparatus thus makes it possible toobtain a fairly high stretching rate. Since the ratio of the surface ofthe stretched film to the surface of the apparatus amounts to 0.645,only a small amount of space is wasted.

The apparatus, whose frame is arranged horizontally, is provided with acentral bevel gearwheel arranged on a vertical axis and with 64 teeth,having a module of 1.25, meshing simultaneously on 12 horizontal-axisbevel gearwheels having 14 teeth which are each secured to ascrew-threaded rod of l0 mm. diameter, with a l.5 mm. pitch. One ofthese l2 screwthreaded rods is provided with a rapid coupling devicewhich is connected to a 0.3 hp. geared motor set, the output speed ofwhich can be regulated between 6 and 36 r.p.m. Each screwthreaded roddrives a carriage which is guided by a cylindrical parallel rod with adiameter of 8 mm., the 12 carriages being provided with grippers forholding the film during stretching, each ofthem over a width of mm. anda depth of6 mm.

The apparatus can be placed in a heating chamber with forced hot aircirculation, and with internal dimensions of 400X400 D520 mm., providedwith a double-access door, the internal door being glazed, and also withappropriate lighting. The lateral wall of the heating chamber is formedwith an aperture permitting the passage of the coupling device whichtransmits the movement causing the stretching of the film.

The stretching of a substantially amorphous film of polyethyleneterephthalate is carried out with this apparatus. From a film ofa widthof 94 mm. obtained by extrusion in the molten state, there is cut a dischaving a diameter of 90 mm., and a square having sides of 50 mm.,centered on this disc, is drawn without scoring the disc. The thicknessof the film is measured for stretching at eight marked points situatedat the apices (A,B,C,D) and the centers (E,F,G,H) of the sides ofthesquare. (The thicknesses measured are given in the table hereinafter).Then the disc is placed between the jaws of the i2 grippers which areclamped equally, with the inner faces of the grippers disposed on a 78mm. diameter circle.

The stretching apparatus carrying the film is then put within a heatingchamber of the type mentioned hereinbefore, the temperature ofwhich haspreviously been regulated to 100 C. After about 10 minutes thetemperature in the heating chamber is stabilized at about 100 C. and thetemperature measured in contact with the film at about 90 C. Stretchingis then begun, the geared motor set driving one of the screwthreadedrods at approximately 33 rpm. Through the glazed door of the heatingchamber. the stretching of the film gripped gradually at a speed of 50mm. per minute, until one of the grippers reaches an electricalend-of-travel contact (or the operator himself stops the stretchingoperation), after 129 seconds. The internal faces of the grippers andthen disposed on a 28! mm. diameter circle which is reduced to 280 mm.in order to prevent any risk of inopportune tearing.

The thermal fixing of the film is then carried out in order to releaseits internal tensions, transferring the apparatus quickly to a secondheating chamber similar to the first, which has previously been broughtto a temperature of C. It is left thus for 16 seconds and then removedand, after cooling, the grippers are unclamped and the stretched,stabilized film is released, and is substantially in the form of al2-sided polygon. The square which had been drawn has been stretched toa square with a side length of mm. without any distortion beingdetectable, which shows that stretching has been effected uniformly overthe greater portion of the surface of the disc. A measurement of thethickness of the film at the eight points on the square which weredefined hereinbefore gives the values shown in the following table. (Thethickness differences noted are due substantially to the thicknessvariations of the film before stretching).

Thickness of the film in microns at the various marked points Marked A BC D E F G H points Before Stretching 335 320 330 305 330 305 310 340After Stretching 25 25 22 I8 25 I8 25 25 The stretching amounts obtainedare:

a. along any diameter of the circles contacting the grippersT,,=lO0[(28078)/78 l=259percent b. along any side of the square drawn onthe disc c. in the thickness of the film where (F322 u: mean initialthickness ef=22.5 ,u: mean final thickness.

It will be noted that the stretching amounts T and T,, are near to oneanother in value, the stretching amount T is smaller since thestretching has been less considerable locally in the vicinity of thegrippers.

Then the stretched film is subjected to a tensile test in two directionsperpendicular to one another in order to determine:

a. its resistance to rupture R b. its elongation A at the moment ofrupture.

The following results are obtained:

in a first direction:

1\=kg./cm. A=l ,600 60 percent in a direction perpendicular to the firstdirection R=1,780 kg/cm A=6O percent The values found are substantiallyof the same order of magnitude, taking into account the irregularitiesin thickness of the film before stretching. The film is thus stretchedin a substantially isotropic manner.

EXAMPLE 2 An apparatus capable of stretching films of plastics materialis constructed which is shown in F I68. 5 and 6, the dimensions being asfollows: diameter 360 mm.. height. 65 mm./95 mm.

of circular shape of a diameter of at least 90 mm. and stretches them incircles reaching 290 mm. in diameter. The ratio of the film surfacesafter and before stretching is as much as 10. and the apparatus thusmakes it possible to obtain fairly high stretching amounts. Since theratio of the surface of the stretched film to the surface of theapparatus reaches 0.645, the bulk of the apparatus is not considerablein comparison with the surface of the stretched film.

The apparatus, whose frame (1) is arranged horizontally, is providedwith 12 carriages each sliding by means of bronze sleeves along twoslideways constituted by cylindrical steel rods having a diameter of 8mm. Each carriage is driven by a screwthreaded rod having a diameter of10 mm., a pitch of L5 mm., secured to a bevel gearwheel having 18 teethand a module of L25. One of these 12 screw-threaded rods is providedwith a rapid coupling device which is connected to a 0.3 hp. gearedmotor set and an output speed regulatable between 6 and 36 rpm. The l2carriages are provided with grippers for holding the film duringstretching. each over a length of 20 mm and a depth of 6 mm. Machiningof the faces of the hub on which the gearwheels bear is effected to aprecision of 0.005 mm., and the gearwheels are cut with the sametolerances and mesh with one another substantially without play. Therespective angular positions of the screw-threaded rods and theslideways are defined with a precision equal to 12 seconds of an arc.The carriages are displaced on slideways by means of screw-threaded rodswhose parallelism is regulated to within 0.01 mm. These can expandfreely at the rim side by 0.2 mm.

The stretching of a film of polyethylene terephthalate is carried out asin the preceding example and under the same conditions.

It is found that, after having introduced the stretching apparatus intothe heating chamber, the temperature in the chamber is stabilized atabout 100 C. and the temperature measured in contact with the film atabout 90 C. at the end of only 3 minutes. The stretching lasts 130seconds and at the conclusion of stretching the internal faces of thegrippers define a circle with a diameter of 226 mm. Measurements, beforeand after stretching, of the thickness of the film at the apices of thesquare drawn before stretching give the following values:

Thickness of the film in microns at the various it is noteworthy thatthe stretching with this apparatus has cancelled the considerableinitial thickness inequalities and produces a sheet of a substantiallyuniform thickness.

The following stretching amounts are obtained:

a. on any diameter of the circles contacting the grippers:

T,,=l00[(22677)77]=l94 percent b. on any side of the square drawn on thedisc:

T,,=l00[(l 8050)50]=260 percent c. calculated in accordance with themean thicknesses of the film before and after stretching:

The stretched film is finally subjected to a tensile test again as inexample 1'. the following results are obtained: Y

It will be noted that apart from the breaking load which shows thepartially crystalline state of the stretched film, the elongation andthe modulus of elasticity are of the same order of magnitude. Therefore,the film is stretched in a substantially isotropic manner.

We claim:

1. Apparatus for isotropically stretching a noncircular sheet ofplastics, said apparatus comprising in combination:

a. a central hollow hub;

b. a rim coaxial with said central hollow hub;

c. a plurality of separate screw-threaded rods arranged perpendicularlyto the axis of said hollow hub and extending from said hollow hub tosaid rim, each rod being rotatable about its own axis;

d. a plurality of slideways arranged perpendicularly to the axis of saidhollow hub parallel to and in close proximity to said screw-threadedrods;

e. a plurality of screw-threaded carriages, each threadably engaging onone of said screw-threaded rods, and slidable on one of said slideways;

f. gripping means for gripping the sheet mounted on each carriage, saidgripping means being disposed in an array of a particular configuration;and

g. gearing, interconnecting said screw-threaded rods to cause rotationof each of the remaining rods, the pitches of the threads of said rodsbeing such that at least one of said gripping means moves at a ratedifferent from at least one of the remaining gripping means so as tomaintain said configuration constant.

2. Apparatus as specified in claim 1, having at least fivescrew-threaded rods and carriages.

3. Apparatus as specified in claim 1, wherein a bevel gear is mounted atthe inner end of each of said rods, wherein a single bevel gear ismounted on said hollow hub, and wherein said single bevel gearcooperates with each of the bevel gears mounted on said rods.

4. Apparatus as specified in claim 1, wherein a bevel gear is mounted atthe inner end of each of said rods, and each of said bevel gearscooperates with the bevel gears on each of the adjacent rods, the pitchofthe screw thread of each rod being oppositely directed to the pitch ofthe thread of each adjacent rod.

5. Apparatus as specified in claim 1, wherein a bevel gear is mounted atthe inner end of each rod and wherein two idler gears are provided foreach rod, each idler gear cooperating with the bevel gears on each ofthe adjacent rods.

6. Apparatus as specified in claim 1, wherein said configuration is asquare, and the pitches of the threads of said rods are such that saidsquare configuration is maintained.

1. Apparatus for isotropically stretching a noncircular sheet ofplastics, said apparatus comprising in combination: a. a central hollowhub; b. a rim coaxial with said central hollow hub; c. a plurality ofseparate screw-threaded rods arranged perpendicularly to the axis ofsaid hollow hub and extending from said hollow hub to said rim, each rodbeing rotatable about its own axis; d. a plurality of slideways arrangedperpendicularly to the axis of said hollow hub parallel to and in closeproximity to said screw-threaded rods; e. a plurality of screw-threadedcarriages, each threadably engaging on one of said screw-threaded rods,and slidable on one of said slideways; f. gripping means for grippingthe sheet mounted on each carriage, said gripping means being disposedin an array of a particular configuration; and g. gearing,interconnecting said screw-threaded rods to cause rotation of each ofthe remaining rods, the pitches of the threads of said rods being suchthat at least one of said gripping means moves at a rate different fromat least one of the remaining gripping means so as to maintain saidconfiguration constant.
 2. Apparatus as specified in claim 1, having atleast five screw-threaded rods and carriages.
 3. Apparatus as specifiedin claim 1, wherein a bevel gear is mounted at the inner end of each ofsaid rods, wherein a single bevel gear is mounted on said hollow hub,and wherein said single bevel gear cooperates with each of the bevelgears mounted on said rods.
 4. Apparatus as specified in claim 1,wherein a bevel gear is mounted at the inner end of each of said rods,and each of said bevel gears cooperates with the bevel gears on each ofthe adjacent rods, the pitch of the screw thread of each rod beingoppositely directed to the pitch of the thread of each adjacent rod. 5.Apparatus as specified in claim 1, wherein a bevel gear is mounted atthe inner end of each rod and wherein two idler gears are provided foreach rod, each idler gear cooperating with the bevel gears on each ofthe adjacent rods.
 6. Apparatus as specified in claim 1, wherein saidconfiguration is a square, and the pitches of the threads of said rodsare such that said square configuration is maintained.